Parametric acoustic source beamwidth control     

Konrad  W. Moffett  M. 《Oceanic Engineering, IEEE Journal of》1978,3(3):57-59

In addition to its narrow beamwidth and wide-band capability, the underwater parametric acoustic source has the advantage of maintaining a relatively constant beamwidth over a wide range of difference frequencies. However, for some applications, the beamwidth of the difference frequency is narrower than desired or an adjustable beamwidth is required. This article describes techniques by which the beamwidth at a given difference frequency can be increased and varied over a 3-to-1 range by controlling the input waveform and amplitude to the transducer. The effect of the waveform changes on the harmonic content of the difference-frequency energy is also discussed.  相似文献   

Electron heating resulting from interhemispherical transport of photoelectrons     

W.E. Swartz  G.J. Bailey  R.J. Moffett 《Planetary and Space Science》1975,23(4):589-598

Ambient electron heating rates along several magnetic field lines have been determined for subsequent studies of electron and ion temperatures. Use is made of the modified diffusion method for computing the heating of the ambient plasma, and the escape fluxes from both hemispheres are coupled by self-consistent upper boundary conditions supplied by interhemispheric fluxes degraded in energy along the magnetic field tubes. Heating rates and fluxes are presented for several low L-shells appropriate for noon solstice conditions when both hemispheres are illuminated. The opacity of the field tubes as a function of L is expected to go through a minimum due to the transition from large collective effects of coulomb small angle scattering and energy loss for high L-shells, to a domination by neutral scattering all along the field lines of low L-shells.  相似文献   

A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere     

R. L. Balthazor  R. J. Moffett  G. H. Millward 《Annales Geophysicae》1997,15(6):779-785

First results of a modelling study of atmospheric gravity waves (AGWs) are presented. A fully-coupled global thermosphere-ionosphere-plasmasphere model is used to examine the relative importance of Lorentz forcing and Joule heating in the generation of AGWs. It is found that Joule heating is the dominant component above 110km. The effects of the direction of the Lorentz forcing component on the subsequent propagation of the AGW are also addressed. It is found that enhancement of zonal E × B forcing results in AGWs at F-region altitudes of similar magnitudes travelling from the region of forcing in both poleward and equatorward directions, whilst enhancement of equatorward meridional E × B forcing results in AGWs travelling both poleward and equatorward, but with the magnitude of the poleward wave severely attenuated compared with the equatorward wave.  相似文献   

Modelling high-latitude electron densities with a coupled thermosphere-ionosphere model     

J. Schoendorf  A. D. Aylward  R. J. Moffett 《Annales Geophysicae》1997,14(12):1391-1402

A few of the difficulties in accurately modelling high-latitude electron densities with a large-scale numerical model of the thermosphere and ionosphere are addressed by comparing electron densities calculated with the Coupled Thermosphere-ionosphere Model (CTIM) to EISCAT data. Two types of simulations are presented. The first set of simulations consists of four diurnally reproducible model runs for a K_p index of 4o which differ only in the placement of the energetic-particle distribution and convection pattern input at high latitudes. These simulations predict varying amounts of agreement with the EISCAT data and illustrate that for a given K_p there is no unique solution at high-latitudes. Small changes in the high-latitude inputs cause dramatic changes in the high-latitude modelled densities. The second type of simulation consists of inputting statistical convection and particle precipitation patterns which shrink or grow as a function of K_p throughout a 3-day period 21–23 February 1990. Comparisons with the EISCAT data for the 3 days indicate that equatorward of the particle precipitation the model accurately simulates the data, while in the auroral zone there is more variability in the data than the model. Changing the high-latitude forcing as a function of K_p allows the CTIM to model the average behavior of the electron densities; however at auroral latitudes model spatial and temporal scales are too large to simulate the detailed variation seen in individual nights of data.  相似文献   

SO2 from episode 48A eruption,Hawaii: Sulfur dioxide emissions from the episode 48A East Rift Zone eruption of Kilauea volcano,Hawaii     

RJ Andres  PR Kyle  JB Stokes  WI Rose 《Bulletin of Volcanology》1989,52(2):113-117

An SO₂ flux of 1170±400 (1) tonnes per day was measured with a correlation spectrometer (COSPEC) in October and November 1986 from the continuous, nonfountaining, basaltic East Rift Zone eruption (episode 48A) of Kilauea volcano. This flux is 5–27 times less than those of highfountaining episodes, 3–5 times greater than those of contemporaneous summit emissions or interphase Pu'u O'o emissions, and 1.3–2 times the emissions from Pu'u O'o alone during 48A. Calculations based on the SO₂ emission rate resulted in a magma supply rate of 0.44 million m³ per day and a 0.042 wt% sulfur loss from the magma upon eruption. Both of these calculated parameters agree with determinations made previously by other methods.  相似文献   

Simulations of the seasonal and universal time variations of the high-latitude thermosphere and ionosphere using a coupled,three-dimensional,model   总被引：1，自引：0，他引：1  

T. J. Fuller-Rowell  D. Rees  S. Quegan  R. J. Moffett  G. J. Bailey 《Pure and Applied Geophysics》1988,127(2-3):189-217

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Coupling between the F-region and protonosphere: Numerical solution of the time-dependent equations     

R.J. Moffett  J.A. Murphy 《Planetary and Space Science》1973,21(1):43-52

This paper describes a new method of solution of the time-dependent continuity and momentum equations for H⁺ and O⁺ in mid-latitude magnetic field tubes from the F-region to the equator. For each ion the equations are expressed as an integro-differential equation. This equation is treated as an ordinary differential equation and solved by a searching method. By means of this method, the distribution of H⁺ in the O⁺?H⁺ transition region and the protonosphere can be investigated and the influence of H⁺ fluxes on the F layer examined.As an example of application of the method a suggestion by Park (1971) about observed night-time enhancements of N_mF2 is examined. He suggested that lowering of the F layer some hours after a magnetic substorm may cause N_mF2 to increase because of increased ion influx from the protonosphere. In the present calculations the Flayer is maintained around a constant height for some time and then abruptly lowered. Under the conditions adopted the resulting increase in downward H⁺ flux is sufficient to maintain N_mF2 against the increased recombination but not to increase N_mF2 significantly. It is emphasised that these results are not conclusive.  相似文献   

A study of atmospheric gravity waves and travelling ionospheric disturbances at equatorial latitudes     

R. L. Balthazor  R. J. Moffett 《Annales Geophysicae》1997,15(8):1048-1056

A global coupled thermosphere-ionosphere-plasmasphere model is used to simulate a family of large-scale imperfectly ducted atmospheric gravity waves (AGWs) and associated travelling ionospheric disturbances (TIDs) originating at conjugate magnetic latitudes in the north and south auroral zones and subsequently propagating meridionally to equatorial latitudes. A fast dominant mode and two slower modes are identified. We find that, at the magnetic equator, all the clearly identified modes of AGW interfere constructively and pass through to the opposite hemisphere with unchanged velocity. At F-region altitudes the fast AGW has the largest amplitude, and when northward propagating and southward propagating modes interfere at the equator, the TID (as parameterised by the fractional change in the electron density at the F2 peak) increases in magnitude at the equator. The amplitude of the TID at the magnetic equator is increased compared to mid-latitudes in both upper and lower F-regions with a larger increase in the upper F-region. The ionospheric disturbance at the equator persists in the upper F-region for about 1 hour and in the lower F-region for 2.5 hours after the AGWs first interfere, and it is suggested that this is due to enhancements of the TID by slower AGW modes arriving later at the magnetic equator. The complex effects of the interplays of the TIDs generated in the equatorial plasmasphere are analysed by examining neutral and ion winds predicted by the model, and are demonstrated to be consequences of the forcing of the plasmasphere along the magnetic field lines by the neutral air pressure wave.  相似文献   

Environmental controls on blowing dust direction at lubbock,Texas, U.S.A.     

Jeffrey A. Lee  K. Evan Moffett  B. L. Allen  Richard E. Peterson  James M. Gregory 《地球表面变化过程与地形》1994,19(5):437-449

Blowing dust is a common phenomenon at Lubbock, Texas, on the Southern High Plains. The directional variability of blowing dust estimated with the ‘sand rose’ technique, using wind speed and direction data, suggest that dust transport occurs from all directions. An empirical method of determining directional variability using meteorological data on visibility reductions due to blowing dust, however, indicates that most dust comes from the west and south-west. In addition to wind speed and direction, other environmental factors must be considered in explaining the spatial pattern of dust transport. Soil erodibility is variable in the region, with the most extensive area of highly erodible soils to the west and south-west of Lubbock. Rangeland dominates land use to the east, while agriculture is extensive to the north, west and south. Local farming techniques leave bare soil during the winter and spring, when most airborne dust is produced and also when strong winds are common from the west and south-west. Soil moisture is lowest to the southwest of Lubbock, which leads to a decrease in soil structure and an increase in the potential for wind erosion in that direction. Relative humidities affect threshold wind speeds and are lowest during March and April, when winds are common from the west and south-west. The spatial and seasonal variability and interactions between many factors, both natural and human-controlled, must therefore considered in explaining the directional variability of aeolian sediment transport at Lubbock.  相似文献   

The effect of disturbed-time electric fields on the inner plasmasphere     

H. F. Balmforth  R. J. Moffett  A. J. Smith  G. J. Bailey 《Annales Geophysicae》1994,12(4):296-303

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